Role of oospores as primary inoculum for epidemics of downy mildew caused by Peronospora arborescens in opium poppy crops in Spain

Abstract

This study investigated the role of Peronospora arborescens oospores as primary inoculum for downy mildew of opium poppy and infection types that they may give rise to in Spain using an integrative experimental approach that combined pathogenicity tests in growth chambers and field microplots, together with molecular detection of P. arborescens infection by specific nested-PCR assays. The results demonstrated that oospores in infested soil or leaf debris were effective inoculum for ingress of the pathogen through underground plant tissues early in poppy seedling growth. This gave rise to systemic infections that reproduced the stunting, chlorotic syndrome frequently observed in affected plants in commercial fields. Additionally, infection of underground tissues of older plants by oospore inoculum could remain asymptomatic. Results also suggested that sporangia formed on infected plants are effective in producing secondary local infections that later may become systemic and either symptomatic or asymptomatic. Finally, and more importantly, those delayed symptomatic or asymptomatic systemic infections, as well as secondary local infections of capsules, can give rise to infected seeds. This research on the biology of P. arborescens on poppy plants and epidemiology of downy mildew may help to develop knowledge-based disease-management strategies of use in the protection of yields of opium poppy crops in Spain and elsewhere.

Short communication: Local infection of opium poppy leaves by Peronospora somniferi sporangia can give rise to systemic infections and seed infection in resistant cultivars

“…More interestingly, use of the nested-PCR assay allowed detection of P. somniferi in 20 to 100% seed samples from symptomless capsules, as well as from capsules of all the genotypes rated as resistant due to the absence of DM symptoms. Comparatively, use of primers OMPac7fw/OMPac7rv in single PCR assays allowed detection of the pathogen in 50 to 75% of seed samples in only three of the nine resistant genotypes evaluated (Table 1) Therefore, results show that the 'drop' inoculation of opium poppy leaves with a suspension of P. somniferi sporangia is efficient for establishing infection and reproducing the chlorotic and deformation syndrome frequently observed in DM affected-plants in commercial fields (Montes-Borrego et al, 2009a). This 'drop' inoculation method would be particularly useful when assessing a large number of opium poppy varieties under field conditions, since the amount of inoculum needed is 40X times lower than that for the spray inoculation method used previously (data not shown).…”

“…Seeds were proven free of infection by P. somniferi by molecular testing using a specific-PCR assay (Montes-Borrego et al, 2009a). Out of the 20 genotypes, nine (designated R1-R9) and five (designated S1-S5) genotypes had previously been characterized as potentially resistant and susceptible to P. somniferi, respectively, in field experiments under natural inoculum pressure (Montes-Borrego et al, unpublished results), three genotypes (designated M8, EA and TAR) are commercial varieties, and the remaining three genotypes (designated F1-1, F1-2 and F1-3) are F1 progenies from crosses between TAR and R9 genotypes.…”

“…Briefly, primer DC6 specific for species in the orders Pythiales and Peronosporales of the Oomycota and the universal ITS4 primer were used in the first round of a nested-PCR protocol. The resulting PCR product was diluted 1:10 and used as a template for a second round of amplification using primers OMPac7fw/OMPac7rv and PCR conditions described by Montes-Borrego et al (2009a). Results of the nested-PCR were compared with those of single PCR using primers DC6/ITS4 and OMPac7fw/OMPac7rv only.…”

“…However, the taxonomic status of the latter two species was recently clarified by means of molecular analyses in conjunction with morphological characteristics (Voglmayr et al, 2014). In Spain, the incidence and severity of DM attacks caused by P. somniferi have steadily increased every year since it was first detected in 2005, thus making this disease the main limiting factor for opium poppy producers (Montes-Borrego et al, 2009a). In Tasmania (Australia), the largest world producer of poppy straw (INCB, 2014), DM caused by P. meconopsidis have affected opium poppy crops for decades (Scott et al, 2004).…”

“…According to findings from previous research, diseased plant parts, infested soil and infected seeds are the main sources of primary inoculum (oospores) for P. somniferi that can give rise to systemic infection of the plant (Montes-Borrego et al, 2009a). Conversely, P. somniferi sporangia formed on infected leaves are thought to be more important as secondary inoculum for leaf infections, with a primary role in the development of polycyclic DM epidemics (Navas-Cortés et al, 2009;Calderón et al, 2014), as it occurs for the other DM pathogen, P. meconopsidis, in Tasmania (Scott et al, 2003;.…”